The present disclosure relates to solid state lamp control, and in particular to control of a solid state lamp when coupled to a trailing edge phase cut dimmer.
Solid state lamps, such as Light emitting diode (LED) based lamps have become commonplace. As used herein, a “lamp” may also be referred to as a “light bulb”. As shown in FIG. 1, in a solid state lighting system a solid state lamp (SSL) 102 may be connected to an AC mains supply 102 via a phase cut dimmer switch 104.
Phase cut dimmer switches comprise a phase cut switching element that operates to disconnect the AC mains from the lamp at a given phase angle of every AC half cycle. The amount of phase cut (portion of the AC half wave that is removed) can be adjusted based on a dimming setting that is selected by an end user.
There are two types of phase cut dimmer switches that are typically in use, leading edge and trailing edge. In a leading edge phase cut dimmer switch, the phase cut starts at or just after a zero crossing point of the AC mains signal and is maintained until the AC mains signal reaches its set phase angle. In a trailing edge phase cut dimmer switch the AC mains signal is disconnected from the lamp at the set phase angle and until the next zero crossing of the AC mains signal.
A common phase cut switching element is a bidirectional triode thyristor, more commonly referred to as a triac. This is a switching device that can conduct current in either direction when it is triggered. However, once a triac has been switched on and settled it requires a minimum holding current in order to maintain its state. This means that the lamp must maintain a minimum load to ensure correct operation of the triac. If the minimum current threshold is not met, the triac device resets and prematurely goes to the off state, causing noticeable light flicker or complete failure.
However, in the case of LED-based lamps, maintaining a minimum load while the triac is in the conducting state is difficult, primarily due to the much greater operational efficiency of LED-based lamps.
Furthermore, the minimum holding current varies widely between triac devices. This further complicates the design of LED-based lamps, because the variation increases the risk that the lamps may be incompatible with certain dimmers.
In order to ensure the triac device remains in the on state as defined by the dimming setting, existing LED-based lamps contain what is commonly referred to as a bleeder circuit.
The bleeder circuit adds to the over-all load that the lamp draws from the AC mains, providing the necessary current to keep the triac device in the on state at the desired periods.
FIG. 2 shows an example of a prior art LED-based lamp comprising a DC voltage source, power stage, output stage, and bleeder. The topology of FIG. 2 is commonly referred to as a non-isolated buck-boost circuit. An input stage receives and rectifies an incoming AC voltage, in this case comprising a diode bridge DB1 and bulk capacitor C1 to provide an unregulated DC voltage source. Inductor L1, power switch S1 and controller U1 comprise a power stage. The power switch S1 may be a transistor. When the power stage is enabled, the power switch S1 switches on an off rapidly. Output regulation is provided by controller U1 controlling the ON and OFF cycles of power switch S1. An output stage comprises rectifier diode D1 and output capacitor C2. A light emitter LED1 is coupled to the regulated output. The light emitter may comprise an LED or an array of LEDs (a single LED is shown for clarity of illustration).
Ballast resistor R1, switch S3 and controller U2 comprise an example of a bleeder circuit. When it is determined that bleeder current is required in order to maintain the proper operation, U2 places S3 in to the ON state, creating a load current across the bulk capacitor through R1. Although this maintains proper operation of the dimmer switch, it is purely dissipative and reduces the operational efficiency of the lamp.
FIG. 3 shows typical operating waveforms of the prior art LED based lamp shown in FIG. 2 coupled to a leading edge phase cut dimmer switch. FIG. 3a plots voltage against time and shows the AC mains voltage waveform (dotted line) superimposed on the phase cut AC input waveform (solid line).
FIG. 3b shows the ON and OFF cycles of power switch S1 over time as controlled by the controller U1 to maintain output regulation. The controller U1 initiates the ON and OFF cycles after the triac of the dimmer switch starts to conduct. Once the correct amount of energy is delivered to the load (that is, the lamp), controller U1 stops the energy transfer to the load by suspending the ON and OFF cycles of S1. This is illustrated by the regulation threshold 300, which can be adjusted in order to maintain output regulation.
When the ON and OFF cycles of S1 are suspended, the load that the LED based lamp places on the AC input voltage is minimal, requiring the bleeder to be enabled for the reasons discussed above. As shown in FIG. 3c, controller U2 enables the bleeder circuit by placing S2 in the ON state when the switching cycles of S1 are disabled. The switching cycles of S1 occur when the energy stored in the bulk capacitor C1 is at a maximum.
FIG. 4 shows typical operating waveforms of the prior art LED based lamp shown in FIG. 2 coupled to a trailing edge phase cut dimmer switch. FIG. 4a plots voltage against time and shows the AC mains voltage waveform (dotted line) superimposed on the phase cut AC input waveform (solid line).
FIG. 4b shows the ON and OFF cycles of S1 over time as controlled by the controller U1 to maintain output regulation. The controller U1 initiates the ON and OFF cycles after the triac of the dimmer switch starts to conduct, which in the case of a trailing edge dimmer switch is at the zero crossing of the AC mains waveform. Once the correct amount of energy is delivered to the load controller U1 stops the energy transfer to the load by suspending the ON and OFF cycles of S1. This is illustrated by the regulation threshold 400, which can be adjusted in order to maintain output regulation.
When the ON and OFF cycles of S1 are suspended, the load that the LED based lamp places on the AC input voltage is minimal, requiring the bleeder to be enabled for the reasons discussed above. As shown in FIG. 4c, the controller U2 enables the bleeder circuit by placing S2 in the ON state when the switching cycles of S1 are disabled.
In contrast to the case of the leading edge dimmer, the bleeder is enabled when the amount of energy in the bulk capacitor is at its maximum. Since the energy dispensed by the bleeder circuit is purely dissipative, the operational efficiency of the lamp is greatly reduced.